1. Field of Invention
This invention relates to power amplifiers and in particular to such amplifiers which are designed to operate into a low impedance, at a low frequency, and at a relatively high supply voltage.
2. Background Description
Amplifiers such as are disclosed in the instant application may be used in many different circuits and applications; however, only its use in a ringing generator will be discussed in detail here, as this was the device shosen for the reduction to practice. It is to be understood that other uses would easily come to mind.
A ringing generator supplies the ringing signal over telephone lines from a central office to a subscriber's telephone set in order to operate the ringer located therein. Typically, the ringing voltage is in the order of 90 V rms, 20 or 25 Hz signal which operates into a low impedance offered by the combination of a plurality of telephone lines and telephone set ringers. Because the ringing generator chosen will be called upon to supply ringing power to a given number of telephone sets at the same time, this ringing generator is capable of supplying about 15 watts continuously. However, this power level does not represent a constraint upon the output power. A number of different techniques have been used in the past to generate the ringing signal.
One such technique is to use the classic self-oscillating power inverter configuration. A disadvantage of this type of device is that it delivers a squar wave signal and at a fixed frequency. Other techniques include the straight class B amplifier which gives good efficiency but requires twice the peak-to-peak current as is required by the instant invention; and the class B amplifier bridge, which reduces the current requirement but is less efficient than the configuation of the instant invention. These latter two circuits will be considered in more detail and compared with that of the instant invention under the description of the invention.